Percutaneously-inserted intravenous catheters are used in neonates for a number of reasons including parenteral nutrition, infusing drugs and taking blood for example. When carrying out such procedures the end of the catheter is inserted into a vein and positioned in close proximity to the heart in the vena cava. In some procedures performed shortly after birth, an umbilical vein is used.
The current procedure for inserting a catheter uses a catheter marked with graduations. To insert the catheter marked with graduations into a neonate involves measuring the baby and applying a formula to determine to which graduation the catheter should be inserted and then taking X-ray images of the baby to check the position of the catheter. If the catheter has been inserted too far it can be withdrawn slightly. However, if the catheter has not been inserted sufficiently far it cannot be inserted further due to loss of the sterile insertion field when performing the X-ray.
Using a procedure which involves taking X-ray images of a neonate is not desirable for a number of reasons. First, exposure to X-rays can itself lead to harm. Secondly, it is necessary to disturb or move the neonate from one location to another to perform the X-ray. Such movement of an ill neonate can cause distress leading to clinical deterioration, and also may involve delay. Thirdly, the very fine gauge intravenous lines used in neonatal practice may be difficult to localise accurately on X-ray, even with the use of contrast media.
Another known procedure involves making a catheter one of the electrodes of an electrocardiogram (referred to hereinafter as an “ECG”) monitor by filling the catheter with electrically conductive saline and attaching a Vygon® connector or similar to the catheter. The idea behind the Vygon® connector is that with the catheter acting as an electrode, as it moves to the correct position with respect to the baby's heart a characteristic pattern is produced on the display of the ECG. Such a procedure is described in a number of academic papers. For example: Ban C. H. Tsui et al, Umbilical vein catheterization under electrocardiogram guidance, Pediatric Anesthesia 2005, 15: 297-300; Hoffman M. A. et al, Central Venous Catheters No X-Rays Needed: A Prospective Study in 50 Consecutive Infants and Children, Journal of Pediatric Surgery, Vol 23, No 12 (December), 1988: pp 1201-1203; Neubauer A-P, Percutaneous central iv access in the neonate: experience with 535 silastic catheters, Acta Paediatr 84: 756-60. 1995; Neubauer A-P, Central Venous Placement of Silastic Catheters by Recording the Intravascular ECG A Prospective Study in 50 Infants Weighing Less than 1000 g, Klin Padiatr 1991, 203: 146-148.
In Hoffman M. A. et al, Central Venous Catheters No X-Rays Needed: A Prospective Study in 50 Consecutive Infants and Children, Journal of Pediatric Surgery, Vol 23, No 12 (December), 1988: pp 1201-1203, central veneous catheterization was performed on infants and children using catheters having internal diameters of 0.025 to 0.040 inches (0.635 mm to 1.016 mm). The catheter was first flushed with 0.9% saline and then filled with 3% saline. An electrical connection was made from the distal end of the catheter to the operating room oscilloscope, replacing the left arm input lead.
In Ban C. H. Tsui et al, Umbilical vein catheterization under electrocardiogram guidance, Pediatric Anesthesia 2005, 15: 297-300, umbilical venous catheters (referred to hereinafter as “UVC”) were inserted from the umbilicus into the thorax, with the optimal position being when the catheter tip was located in the inferior vena cava or at the junction of the inferior vena cava (referred to hereinafter “IVC”) and the right atrium. These experimenters connected a Johans ECG adapter to the distal port of a 3 or 5 French (single or triple lumen) UVC so that the tip of the UVC would become a unipolar ECG electrode. The (external) diameter of a 3 French UVC is 1.00 mm whereas the (external) diameter of a 5 French UVC is 1.7 mm.
In Neubauer A-P, Percutaneous central access in the neonate: experience with 535 silastic catheters, Acta Paediatr 84: 756-60. 1995 the catheters used were Epicutaneo Cava-Catheters (Art. No 2184.005, Vygon Medical Products Aachen, Germany) having a length of 30 cm, an internal diameter of 0.3 mm and an external diameter of 0.6 mm. The catheter was filled with 5.85% NaCl solution.
A problem associated with using the connectors of the Vygon® type is that noise arising from interference, typically mains interference and movement artifacts, prevents an accurate pattern being determined. Excessive noise is produced due to the very high impedance of electrically conductive saline resulting from the very small diameter of the catheter. Neubauer recognized that by increasing the conductive capacity of the saline, this problem could be reduced. He states that through the use of a catheter filled with conducting solution the percentage of radiologically diagnosed malpositions decreased from 64.5% to 9.5%. Furthermore, the interpretation of the ECG tracing presented no difficulty even to the less experienced operator. However, increasing the strength of the saline solution can damage the red blood cells and carries risk of hypernatremia.
In A P Neubauer, Die zentralvenöse Plazierunf des Silastikkatheters durch Ableitung eines intravasalen EKG Eine prospective Untersuchung an 50 Frühgeborenen unter 1000 g, Klin Padiatr, 203 (1991), it is recognized that when small diameter catheters, which must be used in neonates, are connected to an ECG, the output signal is subject to a significant amount of interference when the catheter is filled with physiological saline. Neubauer overcomes this problem by increasing the conductivity of the saline solution, which is achieved by using 5.85% saline solution. Neubauer recognizes that over-supplying sodium and chloride can be harmful to neonates and overcomes this in the smallest of children by first filling the catheter with glucose solution which may be sprayed out of the catheter without any risk in the event of difficulty in inserting the catheter being encountered. The catheter is only filled with 5.85% saline solution just before the desired insertion length is reached.
In neonates it is desirable to use catheters having very small diameters, both external and internal. As recognized by Neubauer, when filled with physiological saline the impedance of such catheters is very high and the electrical signal detected by such a catheter is accompanied by a great deal of noise due to this high impedance.
In each of the above-mentioned academic papers either the catheter is of a substantial diameter when compared to the diameter of catheter used intravenously in a neonate and/or a more conductive rather than physiological saline solution is used when positioning the catheter. In both circumstances the impedance of the saline within the catheter is reduced when compared with the impedance arising when using a catheter of the diameter suitable for intravenous use in a neonate with physiological saline.
The aim of the invention is to allow the catheter to be used as an electrode of an ECG and to produce a signal which is substantially free of noise and which uses physiological saline.